/*! \page Heg2qmcDoc Homogeneous gas


\section description Description

There are two utilities to generate input files for simulations of
homogeneous gases. <tt>heg2qmc</tt> prepares real-valued wave
functions, <tt>heg2qmcc</tt> writes complex wave functions for
proper utilization of the so-called twist averaging.

\subsection heg2qmc heg2qmc (fixed-node input)

Usage: heg2qmc -o &lt;output base&gt; [-bcs]

Takes N_up, N_down and rs from standard input and writes<br>
&nbsp;&nbsp;   &lt;output base&gt;.i.sys<br>
&nbsp;&nbsp;   &lt;output base&gt;.i.slater<br>
&nbsp;&nbsp;   &lt;output base&gt;.i.basis<br>
&nbsp;&nbsp;   &lt;output base&gt;.i.orb<br>
&nbsp;&nbsp;   &lt;output base&gt;.i.jast2<br>
&nbsp;&nbsp;   &lt;output base&gt;.jast2<br>
&nbsp;&nbsp;   &lt;output base&gt;.centers<br>
where i indexes k-points (twists of boundary conditions) and runs from 0 to 3;
0 is the Gamma-point (all 3 directions periodic), 1 is anti-periodic in
one direction, 2 is anti-periodic in two directions and 3 is
anti-periodic in all three directions. These k-points have differing
weights in the twist average, namely

<table>
<tr> <th>k-point index <td>0   <td>1   <td>2   <td>3
<tr> <th>weight        <td>1/8 <td>3/8 <td>3/8 <td>1/8
</table>

The Jastrow factor &lt;output base&gt;.jast2 contains only a short-range
part and is k-point independent. Jastrow factors &lt;output
base&gt;.i.jast2 are k-point dependent as they contain an extra
long-range part expanded in plane waves (cosine functions)
commensurate with given boundary conditions [N. D. Drummond et al.,
<a href="http://dx.doi.org/10.1103/PhysRevB.70.235119">Phys. Rev. B
70, 235119 (2004)</a>].

If -bcs option is given, extra input is taken (number of virtual orbitals), 
and extra files are written (&lt;output&nbsp;base&gt;.i.bcs). The BCS
function is constructed as a special case of \ref Pfaff_wfDoc wave
function. This approach is deprecated as there is a more efficient
way to perform such calculations in QWalk, particularly with the aid
of \ref BCS_wfDoc wave function.

\subsection heg2qmcc heg2qmcc (fixed-phase input)

Usage: heg2qmcc -o &lt;output base&gt;

Takes N_up, N_down, rs and a k-mesh discretization parameter D from
standard input and writes the same files as described above, only the
index i runs over a wider interval. Tetrahedron method is used to
generate irreducible k-points, the actual number of k-points
corresponding to the lowest values of the discretization parameters D
are

<table>
<tr><th>parameter D  <td>1 <td>2  <td>3   <td>4  <td>5
<tr><th>No. k-points <td>4 <td>10 <td>20  <td>35 <td>56
</table>

Averaging weights of the k-points are written to
&lt;output&nbsp;base&gt;.i.sys files as comments.



*/
